Added comments to ItemManager, used MathUtils.

I added comments to my code in ItemManager to make it easier for me to
understand what I did.
I also replaced Math.random() with MathUtils.random() from LibGDX.
Added getChildItemTypes() and countStatus()

I added functions getChildItemTypes() and countStatus() methods to
ItemManager to help with item generation. The idea behind
getChildItemTypes() is that I should keep a collection of the children's
ItemType components so I won't have to get them every coolDownTime has
passed.